1. Field of the invention.
The present invention relates to inductive winding for electrical apparatus such as transformers, reactors and the like in general, and to spirally wound inductive windings of the continuous disc type having impulse voltage distribution improving electrostatic shields in less than all of its disc coil sections in particular.
2. Prior Art
It is well-known that highly inductive windings, such as in iron core transformers and reactors, when exposed to steep wave front impulse voltages, initially exhibit an exponential distribution of voltage drop along the length of the winding with a very high voltage gradient along its first few turns. This extremely non-uniform distribution of voltage is due primarily to the unavoidable distributed capacitance between each incremental part of the winding and adjacent grounded parts such as the core and casing structure associated with the winding. Such ground capacitance is referred to as "parallel" capacitance. Such a winding also possesses another type of distributed capacitance between turns and groups of turns, the sum of such capacitance being in series with winding terminal. This type of distributed capacitance is referred to as "series" capacitance. If series capacitance alone were present, voltage distribution throughout the winding would be substantially uniform and linear, as it would be also if inductance alone were present. Inasmuch as series and parallel distributed capacitances are inherent characteristics of a highly inductive winding, the voltage distribution of impulse voltages applied to such windings is an extremely important design consideration.
The two principal winding configurations used in power transformers of high voltage and current rating are the layer type formed as a cylindrical helix or group of concentric cylindrical helices, and the radial spiral or continuous disc type. In a continuous disc-type winding, each of a plurality of annular coils is wound in a radial spiral, the coils (i.e., radial spiral) being disposed in axial juxtaposition on a linear core and connected, electrically, in a series circuit relation.
It is also well-known that a layer-type winding has a more linear transient voltage distribution than does a continuous disc-type winding, because the series capacitance of a layer winding is large relative to its parallel capacitance. However, for some high voltage applications, the disc type winding is used in order to avoid a high voltage gradient (and consequent heavy insulation) between helical layers at normal operation voltages. Thus, medium and large power, high-voltage transformers often have low-voltage windings of the layer helical or disc types and high-voltage windings of the disc type. In such transformers, the low-voltage winding is commonly located immediately adjacent the core and is surrounded by the higher voltage disc wound winding. Relative to the high-voltage winding, the entire low-voltage winding is approximately at ground potential and the radial space between them, called the "main gap", is an essential design parameter. The radial dimension of the main gap is determined primarliy by two considerations. One is the maximum permissible voltage stress across the main gap at the low, powercircuit circuit frequency and the other is the voltage stress resulting from high-frequency impulse voltages. In practice, the latter consideration often controls the size of the main gap in disc-type transformer windings.
In disc windings with adjacent winding coils, or disc coil sections connected in a series circuit relation (i.e., a continuous disc winding), the non-linearity of coil-to-coil impulse voltage stress usually requires that the first several turns at the high-voltage end be provided with extra insulation. For reasons of economy and size it is desirable to be able to reduce the size of the main gap and to reduce the amount of insulation between disc coils and between coil turns. All of these results may be accomplished if the normally steep exponential inpulse voltage distribution, which particularly characterizes the continuous disc winding, can be favorably modified and brought closer to an ideal uniform linear distribution.
It is known that the transient voltage distribution between axially juxtaposed coils or groups of coils in a disc type winding may be improved by various expedients which increase series capacitance relative to parallel capacitance. One such expedient is to place one or more shielding conductors between coil turns of the disc coil sections of a winding, as illustrated in U.S. Pat. No. 2,905,911 to KURITA. It is also known that these shield conductors, or electrostatic shields, become less effective as the distance from the high potential end of a winding to the electrostatic shield, increases.
Placing electrostatic shields, of the just-mentioned type along the entire length of a disc wound winding is considered poor design practice because of cost and size considerations and such designs are usually avoided. While it is true that more electrostatic shields will, in fact, improve the transient response of a disc wound winding, there is a region in such a winding, which is some calculable distance from a high potential end of same, where a point of diminishing returns is reached. Providing additional electrostatic shields beyond this region of the winding will result in a degree of impulse voltage distribution improvement that is not justified by the penalty that must be paid to obtain this improvement in terms of increased winding size and cost. Normal design practice is to discontinue electrostatic shields beyond this calculable distance. However, discontinuing electrostatic shields other than at the end of a disc wound winding creates problems that would not be present if electrostatic shields were continued throughout its entire length.
If abrupt changes in series capacitance occur when going from that portion of a disc wound winding having electrostatic shields, hereinafter also designated the compensated portion, to that portion of the winding that does not have electrostatic shields, hereinafter also designated the uncompensated portion, this sudden change in series capacitance will result in an unsatifactory impulse voltage build-up at the beginning of the low series capacitance or uncompensated portion of the winding in a manner that is very similar to the unsatisfactory transient voltage build-up that would be present at the high voltage end of the winding if electrostatic shields were not incorporated therein. If possible, such abrupt changes in series capacitance of a disc wound winding should be minimized to, in turn, minimize said unsatisfactory impulse voltage build-up.